Pulse inverting transformer



Feb. 28, 1961 D. A. MACK PULSE INVERTING TRANSFORMER Filed Feb. 20, 1959 INVENTOR.

DICK A. MACK BY MQ- ATTORNEY.

ted sates PULSE .[NVERTIN G TRANSFORMER Filed Feb. 20, 1959, Ser. No. 794,788

'5 Claims. (Cl. 333-97) The present invention relates to electronic circuit components and more particularly to a transformer for use with fast pulse circuitry.

ln electronic circuits of the class utilizing pulses of short duration as information conveying signals, a situation frequently occurs wherein it is necessary to invert the polarity of pulses from the output of a first circuit before the pulses can be satisfactorily utilized by a subsequent circuit.- The fast pulses under consideration are those having a duration generally less than one microsecond. A conventional transformer means cannot satisfactorily function as a polarity inverter for pulses of this very short time duration. While vacuum tube converters may be utilized for this purpose, such apparatus is expensive and cumbersome in comparison with the present invention.

The present invention was developed specifically for convenient insertion in a coaxial line connecting two electronic circuits. in such usage a physically small inverter requiring no connection to an external power source is desired. Preferably, the inverter must not unduly attenuate the signals nor may significant reflections and ripple be created in the inverted signal.

The inventionmakes use of a conducting. cylinder having a signal input coaxial fitting at one end and an output coaxialfitting at the-other end. The central connector of each fitting is connected to the outside shield connector of the opposite: fitting, which arrangement presents a short circuit to direct currentor low frequencysignals, but at high frequenciesprov-ides a ground return path that has sufficient high frequency impedance so that high frequencies are inverted without appreciable attenuation. To increase such impedance, annular cores ofa high magnetic permeability material such .as-iron'may be disposed withinthe conductive cylinder in spaced apart relationship to the coaxial fittings. Theinvention isthus extremely compact, simple, and inexpensive to manufacture. The unit may be readily coupled to standard circuitry and requires no operating power or connections other than that provided by the pulse input and output fittings.

It is an object of the presentinvention to provide an improved polarity inverter for high frequency electrical pulses.

Itis another object of the invention to provide a pulse inverter having small physical size and having provision for convenient insertion in a coaxial line.

It is still another object of the invention to provide a simple, economically manufactured high frequency pulse transformer for use as a polarity inverter.

It is another object to provide a pulse inverting transformer having a low insertion loss and causing minimum reflection and ripple of the signal.

It is a further object of the invention to provide novel apparatus equivalent to a single turn transformer and characterized by means providing high impedance in such transformer. I

The invention, both as-to its organization and method atent ice of operation, together with further objects and advana cylindrical outside casing 11 composed of a conducting material such as copper. A first circular end closure 12 and a second circular end closure 13 are secured at the ends of the casing 11, such closures 12'and 13 being composed of a conductive metal and being electrically continuous with the casing 11. A first coaxial connector or fitting 14 having an outer shield 16 and a central conductor 17 extends through a suitable opening 18 in the first end closure 12. A conventional insulation means 20 is included in the connector 14 to retain the central conductor 17 in insulated relationship coaxially within outer shield 16. The coaxial connector may be any one of various types widely available and preferably has a characteristic impedance matching that of a connecting coaxial line 19 as shown in phantom. A second coaxial connector or fitting 21 having an outer shield 22 and a central conductor 23 extends through an opening 24 in the second end closure 13, such opening 24 being off-set from the center of second end closure 13 to provide for a unique form of electrical connection which will hereinafter be described.

Referring now to Fig. 2, there is shown in cross section the casing llenclosing the second end closure 13. The off-set opening 24 in the second end closure 13 permits the second coaxial connector 21 to extend :therethrough so that suitable electrical connections may be made to the outer shield 22 and central conductor 23.

Referring again to Fig. l, the second coaxial connector 21 is similar to the first coaxial connector 14 although other types of connectors might be utilized, depending on the type of communicating connector used on an output coaxial line 26 as shown in phantom. The first coaxial connector 14 and the second coaxial connector 21 are secured to the first end closure 12 and second end closure 13 respectively by conventional screws 27. Any

other securing means may be readily employed providinga suitable electrical connection is established between the outer shields 16 and 22 and the first and second end closures 12 and 13, respectively. The outer shields of the coaxial cables 19 and 26, outer shields 16 and 22 of connectors 14 and 21, first and second end closures 12 and 13, and the casing 11 will accordingly all be at electrical ground potential or some other fixed reference voltage.

The dimensions selected for a particular inverter transformer are influenced by the characteristic impedance of the coaxial line with which it is used and the first and second coaxial connectors 14 and 21 have dimensions determined by such considerations. A coaxial transmission line 28 having central conductor 29 and outer shield 31 is disposed coaxially within the cylindrical casing 11, the central conductor 29 and shield 31 being connected to the central conductor 17 and outer shield 16 respectively of the first connector 14. The transmission line 28 extends through the casing 11 towards the second connector 21, but terminates at a point spaced axially inward therefrom whereby a gap 32' is left therebetween.

Considering nowthe unique form of coupling between the two connections, the outer shield 31 and the central conductor 29 are connected to the central receptacle 23 and outer shield 22 respectively of the secondconnector- 21. The oflf-set positioning of the second connector 21, as previously described, permits holding of more uniform spacing between conductors so that the cable characteristic impedance is more easily maintained, thereby reducing reflections and ripple of signals.

As is obvious, the inverter transformer presents a short circuit to a direct current or a low frequency signal. A signal applied to the central conductor 17 of the first connector 14 would follow a path along the central conductor 29 to the outer shield 22 of the second connector 21 and back through the casing 11 to the outer shield 16 of the first connector 14. However, the return portion of the signal path through the casing 11 can be made to have high impedance at high frequencies. The casing 11, together with the transmission line shield 31, may be considered as a second transmission line coupling pulses to the outer shield of the first connector 14 and being effectively in parallel with the output transmission line 26 connected to the second connector 21. If the impedance of such second transmission line is sufficiently high with respect to the impedance of the output transmission line 26, inverted high frequency signals will be present at the center connection 23 of the second connector 21. To achieve this desired condition a plurality of annular ferrite iron cores 33 are disposed around the central transmission line 28 so that the space between the inner and outer conductors of the outer return path transmission line is largely filled with a high impedance inductive material. The cores 33 when placed together effectively form a cylinder of iron core material. Such cylinder could be a single entity; however, the usage of a plurality of cores in joining relationship allows more flexibility in designing inverter transformers with differing electrical characteristics by changing the number of cores 33 included in a particular transformer. For fast pulses, the delay in the outer transmission line is sufficient to allow an inverted output signal to appear at the second connector 21. Effectively, the signal transit time between the ends of the inverter is sufliciently long that fast pulse signals occur at the second connector 21 before the return signal can cancel the input pulses.

Considering now the positioning and supporting of cores 33 within the casing 11, an air gap 34 is left between the iron cores and the outer conductor to diminish the distributed capacity in the outer transmission line, since an increase in distributed capacity decreases the impedance of the outer parallel transmission line. Referring now to Fig. 3 there is shown a section view of the inverter transformer taken at line 3-3 of Fig. l. A plurality of retainers 36 are disposed between the casing 11 and the cores 33 for physical support and positioning thereof. The retainers 36 are preferably composed of a foamed plastic or similar insulating material having a low dielectric constant so that distributed capacity is maintained at a low level. Three such retainers 36 are utilized in this embodiment, each comprising a long strip cemented or otherwise secured between the casing 11 and cores 33, such retainers 36 being disposed longitudinally therebetween and being regularly circumferentially spaced thereabout.

A typical inverter transformer designed for use in a coaxial line having a 125 ohm characteristic impedance has a length of approximately five inches and a diameter of one and one-half inches. The amplitude of input pulses is reduced by an insertion loss of approximately 15% with a voltage reflection coeflicient of 13%. Either the first connector 14 or the second connector 21 may receive input signals, the transformerrbeing non-directive.

Many variations in the physical configuration of the inverter transformer are possible within the scope of the invention. For instance, the air gap may be between The connectors may be omitted entirely and the transformer permanently installed in a coaxial line. For inverting pulses of longer duration, wound type cores may be utilized instead of ferrite cores.

While the invention has been disclosed with respect to a single preferred embodiment, it will be apparent to those skilled in the art that numerous variations and modifications may be made within the spirit and scope of the invention and thus it is not intended to limit the invention except as defined in the following claims.

What is claimed is:

1. In an inverter transformer, the combination comprising a pair of coaxial conductor sections having a gap between the ends thereof and each having an inner conductor and an outer shield conductor, said inner conductor of a first one of said coaxial conductor sections being electrically connected with the outer shield conductor of the second one of said coaxial conductor sections and the inner conductor of said second one of said coaxial conductor sections being electrically connected with the outer shield conductor of said first coaxial conductor section, a quantity of high permeability magnetic core material disposed adjacent one of said pair of coaxial conductor sections, and an electrically conductive element connecting said outer shield conductor of said first coaxial conductor section with said outer shield conductor of said second coaxial conductor section, said electrically conductive element extending around and enclosing said quantity of high permeability magnetic material.

2. In a pulse polarity inverting transformer, the combination comprising a first and a second coaxial transmission line section having a gap therebetween and each having a central conductor and a shield conductor, the central conductor of each of said sections being connected across said gap to the shield conductor of the other section, a first coaxial coupling element having an inner and an outer conductor respectively connected to said central conductor and said shield conductor of said first transmission line section at a first end thereof, a second coaxial coupling element having an inner and an outer conductor respectively connected to said central conductor and said shield conductor of said second transmission line section, a cylinder of ferromagnetic material enclosing said transmission line section, and a cylindrical conductive casing enclosing said material and electrically connected to the outer conductors of both said first and second coupling elements.

3. In apparatus as described in claim 2, the combination further characterized by retainer means disposed within said casing and positioned to space said cylinder of ferromagnetic material apart from said casing to provide and air-gap therebetween.

4. In a polarity inverter, the combination comprising a section of coaxial transmission line having a central inner conductor and a shield conductor, a first coaxial ductor and said shield conductor of said line at one end- 2 thereof, a second coaxial connector having an inner conductor and an outer conductor connected respectively to said outer conductor and said inner conductor of said section of transmission line at the second end thereof, said second coaxial connector being spaced from the second end of said transmission line by a gap, said second coaxial connector being disposed with respect to said transmission line with the central inner conductor of said transmission line axially aligned with the outer conductor of said second coaxial connector, an annular cylinder of ferromagnetic material disposed around said transmission. line, and a conductive casing disposed around said ferromagnetic material, a first end of said casing being connected to the outer conductor of said first coaxial connector and the second end of said casing being connected to the outer conductor of said second coaxial connector.

5. In a polarity inverter for fast pulses, the combination comprising a cylindrical conductive casing, a coaxial transmission line section disposed within said casing and having a central conductor and a shield conductor, a first centrally aperturcd end piece secured to and closing one end of said casing, a first coaxial receptacle afiixed to said first end piece and disposed through the aperture thereof, said first coaxial receptacle having an inner connector and an outer connector connected to the central conductor and shield conductor respectively at one end of said coaxial transmission line section, a sec ond apertured end piece secured to and closing the other end of said casing, a second coaxial receptacle affixed to said second end piece and extending through the aperture therein, said second coaxial receptacle having an inner connector and an outer connector coupled to the shield conductor and central conductor respectively of References Cited in the file of this patent UNITED STATES PATENTS 2,669,603 Prache Feb. 16, 1954 2,812,502 Doherty Nov. 5, 1957 2,932,805 Doherty Apr. 12, 1960 2,934,719 Kyhl Apr. 26, 1960 OTHER REFERENCES Rudenberg: The Distributed Transformer, Raytheon Research Division, 1952.

Shulman: Proceedings of the National Electronics Conference, 1956, pages 550-561. 

